Piston pump, method and installation for filtering water

ABSTRACT

The present invention provides a method and an installation for filtering a liquid using a membrane filter device. The technical sector of the invention is the field of manufacturing filter devices, in particular semipermeable membrane devices. In the invention, a pump ( 3, 4 ) for pressurizing a liquid has three pistons ( 24, 25, 26 ) connected to a common shaft ( 23 ) mounted to move in translation in three chambers ( 18   a   , 18   b   ; 19   a   , 19   b   ; 20   a   , 20   b ) that are in alignment; two of said pistons ( 24, 26 ) being identical and connected to the ends of the shaft ( 23 ), each separating one of the chambers ( 18, 20 ) into two cavities ( 18   a   , 18   b   , 20   a   , 20   b ).

The present invention relates to a method and an installation forfiltering a liquid by using a membrane filter device.

The technical field of the invention is that of making semipermeablemembrane filter devices.

The present invention relates more particularly to a method andapparatus for desalinating sea water or brine by reverse osmosis, and tomethods and apparatuses for ultrafiltration of a liquid such as water,e.g. to produce water that is suitable for drinking or for irrigation.

A drawback of installations for filtering sea water in order todesalinate it is low efficiency: the energy consumed to obtain 1 cubicmeter (m³) of desalinated water is about 5 kilowatt hours (kWh) to 10kWh. In order to recover energy from the resulting supersalinated water,a turbine, such as a “Pelton” type turbine can be used, but given thatthe efficiency of a turbine is low, the overall efficiency of theinstallation is improved only a little. Furthermore, such installationsfitted with centrifugal pumps and turbines are expensive, and theirreliability and longevity are relatively poor.

In installations for desalinating sea water by reverse osmosis, thewater to be treated is delivered to the inlet of a filter device at aninlet pressure which is greater than the osmotic pressure of water. As ageneral rule, the pressure with which water is fed to the inlet of thefilter is not less than 25 bars, e.g. it lies in the range about 30 barsto 100 bars, and in particular in the range about 60 bars to 80 bars.The filter delivers both a concentrate of “supersalinated” water and apermeate of desalinated water (at a pressure which is close toatmospheric pressure). The pressure of the concentrate leaving thefilter is generally at a pressure which is only slightly less than thefeed pressure of water for desalination, e.g. it may be at a pressurewhich is lower by about 1 bar to 5 bars, given that the pressure drop inthe filter is small.

In less powerful filter installations, in particular in nanofiltrationinstallations for treating brine, the filter is fed with water to betreated at a pressure of about 10 bars and a concentrate is recovered ata pressure of about 4 bars to 8 bars.

U.S. Pat. No. 3,825,122 describes pumping apparatus for filtering afluid by reverse osmosis, which apparatus comprises a plurality ofcylinders in alignment defining a main fluid pumping chamber, a boosterchamber for recovering energy from the concentrate, and a hydraulicchamber for actuating the apparatus by means of hydraulic fluidpressurized by a pump. Each chamber is provided with a piston that movesin reciprocating translation under the action of a piston rod which iscommon to all of the pistons. Although the object stated in thatdocument is to maintain a constant flow rate of pressurized fluid, thesystem for reversing the travel direction of the rod by usingend-of-stroke sensors controlling distributor valves placed on ductsconnected to the chambers does not enable a continuous flow rate to beensured. That is probably why that apparatus, like all other pistonpumps systems, has not enjoyed effective industrial development forfiltering by reverse osmosis. Filter membranes are extremely sensitiveto variations in pressure and flow rate which cause them to becomeclogged or to break.

U.S. Pat. No. 4,432,876 describes various apparatus seeking to reducefluctuations in water pressure and flow rate at the outlet from a pump:a device for simultaneously varying the volume of the pump chamber andthe volume of an expansion chamber coupled to the pump chamber; twovariants of the apparatus, one with a controlled valve and the otherwith two piloted check valves mounted head to tail, cause those twochambers to be put momentarily into communication while the piston is atthe end of its stroke so as to clip pressure surges due to suddenopening and closing of valves located on the water ducts. That documentalso proposes apparatus comprising three or more pistons, driven by oneor more common crank shafts, and it recommends avoiding machines havingtwo, four, eight, or 16 pistons. In order to make the apparatusdescribed in U.S. Pat. No. 4,432,876 more compact, and to eliminate thedevices for varying chamber volume, U.S. Pat. No. 4,913,809 describespumping apparatus having two pistons interconnected by a rod and drivenby a double-acting hydraulic actuator, with the pressure controlling theposition of a distributor valve provided on the water ducts beingdelivered with a small offset in time.

In spite of those improvements provided to piston pumps, it can be seenthat present-day reverse osmosis installations essentially comprise lowefficiency centrifugal pumps, since piston pump apparatuses are toocomplex and unsuitable for pressurizing water for delivery to membranefilters.

An object of the present invention is to provide a liquid-filteringmethod and installation that are improved.

An object of the present invention is to improve the overall efficiencyof such filter methods and installations.

In a first aspect, the invention consists in proposing water pumpingapparatus comprising at least two pumps, each pump comprising:

at least two chambers in alignment on a longitudinal axis;

at least two pistons respectively mounted to move in reciprocatingtranslation in each of the two chambers; and

a transmission shaft for transmitting forces between the two pistons,which shaft extends in part in each of the chambers and is mounted toslide relative thereto along said longitudinal axis;

the pumping apparatus further comprising an actuator suitable fordelivering the energy required for compressing the water, minus theenergy recovered from the concentrate by said pistons, by causing theshaft to move—usually periodically—in reciprocating translation(sliding) together with the pistons in each of the pumps, and means forcausing the shaft and the pistons of each pump to pause for a prolongedperiod at the end of each stroke, i.e. twice for each period of theperiodic cycle, thus making it possible to avoid or greatly limit anyvariations in the pressure of the water at the inlet to the filter(s).

The pumping apparatus furthermore comprises means for accelerating oneof said two pumps while another one of said two pumps is pausing for aprolonged period at the end of its stroke, thereby enabling theaccumulated flow rate of water delivered by the pumps to the filter(s)to be maintained at a value which is substantially constant.

In the meaning of the present invention, the term “pause” means aduration during which at least one of said pistons, and in general bothpistons of a pump together with the associated shaft, are substantiallystationary; the duration of said pause is such that its ratio to theperiod of the cycle of the shaft (and of the pistons) is generallygreater than 10⁻³; this ratio can rise to very high values, e.g. about0.1 or more, in particular when said two pumps do not both have the samecapacity; under such circumstances, the pause of the larger capacitypump will be of duration longer than the pause of the smaller capacitypump.

Nevertheless, in general, both pumps will have the same capacity andeach will be controlled in such a manner as to perform end-of-strokepauses of durations that are substantially identical for both pumps.

In order to control the slowing down followed by the pause at the end ofa stroke, at least one of the chambers of at least one of the pumps ispreferably fitted with a sensor for sensing the position of the piston(and/or the shaft) and positioned in such a manner as to issue adetection signal before said piston (and/or said shaft) reaches itsend-of-stroke position; this detection signal is transmitted to anelectronic control. unit which responds to receiving said signal bycausing the energy delivered by said actuator of the pump in question tobe stopped.

The driving energy supplied by said actuator is preferably transmittedto the water via a driving hydraulic fluid acting on a “drive” pistonconnected to said shaft in a manner similar to that described in theabove-mentioned patents. The pause in the delivery of driving fluidunder pressure to the drive piston then causes the pause of the pump inquestion.

In another aspect, the invention consists in providing a water pumpingapparatus comprising two pumps, each pump having two chambers inalignment, each receiving a piston movable in translation within thechamber, the two pistons being interconnected by a sliding shaft, saidapparatus further comprising a double-acting hydraulic actuator fordriving each pump and a loop for circulating a pressurized drivinghydraulic fluid, which loop is single and consequently common to all ofsaid hydraulic actuators of the pumping apparatus; the apparatus furthercomprises means for selectively putting each actuator into communicationwith said loop, which means are controlled in such a manner that the sumof the driving hydraulic fluid flow rate delivered to the actuators issubstantially constant over time, such that the sum of the water flowrates delivered by the pumps of the apparatus is substantially constant.

Said common driving fluid circulation loop preferably comprises a singlepump and a single member for measuring the flow rate in said loop.

Said means for establishing selective communication comprise means thatpermanently prevent simultaneous closure of all of the circuits fordelivering driving fluid to the actuators. Consequently, when a fractionof said selective communication means is closed so as to prevent drivingfluid being delivered to one of said actuators, in order to cause thecorresponding pump to perform a pause, at least a fraction of saidselective communication means is open; given that the overall(aggregate) flow rate of driving fluid remains constant, the flow rateof driving fluid delivered to the other actuators fed by said loop isthen increased, thereby causing an acceleration thereof and also of thecorresponding pump(s).

Said means for selective communication essentially comprise valves thatare electrically controlled by the electronic control unit whichreceives signals representing the positions of the pistons of the valvespreferably together with a signal from a flow meter provided in saidcommon loop. Alternatively, the flow meter measuring the overall flowrate of the driving fluid used by the pumping apparatus can be replacedby a flow meter placed on a duct for conveying the water delivered bythe pumps to the membrane filter. It can also be replaced by a plurality(at least two) of flow meters placed on the ducts for conveying oil andconnecting each actuator to the common loop. It can also be replaced byat least one sensor for sensing the travel speed of the shaft sliding inat least one of the pumps, assuming that the various pumps of theapparatus are provided with chambers, sliding shafts, and pistons ofidentical dimensions. Under such circumstances, in order to ensure thatwater is delivered at a total flow rate that is constant, it suffices toensure that the sum of the velocities of the sliding shafts of thevarious pump is maintained permanently at a constant value.

In a preferred embodiment, each pump has a drive piston fixed in themiddle of said sliding shaft; in this embodiment, each of said pumps hasthree pistons and a common sliding shaft for transmitting forces, eachof the pistons being movable in reciprocating translation in arespective cylindrical chamber, the three chambers being in alignment onthe longitudinal axis of the shaft which corresponds to the axis inwhich the pistons move in translation. Both identical end pistons servefirstly to compress the liquid for filtering and secondly to recoverenergy from the concentrate, and they are placed at respective oppositelongitudinal ends of the shaft. The third or “drive” piston (of smallerdiameter) is fixed on the shaft and is at equal distances from the twoends of the shaft. Thus, each of the two end chambers (referred to as“common” chambers) having a respective one of the two end pistons movingtherein is subdivided into two portions or cavities separated by thepiston and of volume that varies depending on the position of thepiston. A first portion of each chamber has a portion of the shaftsliding in the middle thereof and is connected to the membrane filter toreceive the concentrate (super-salinated water). A second portion ofeach chamber is connected to the ducts conveying the liquid forfiltering (salt water). The central chamber in which the drive pistonmoves is connected to the feed and return ducts for driving hydraulicfluid, preferably constituted by oil.

The apparatus of the invention presents advantages:

each of the end pistons which is in contact via a first one of its twofaces (a “front” face) with the fluid to be filtered and which is incontact via a second one of its two faces (a “rear” face) with theconcentrate leaving the filter is subjected to mechanical stresses thatare small, given the small pressure difference that exists between thesetwo liquids; in addition, this small pressure difference does notrequire the pistons to be fitted with sealing rings that are complex andexpensive; and in any event a small amount of leakage through such apiston ring is easily tolerated; and

unlike the apparatus described in U.S. Pat. No. 3,825,122, no portion ofthe shaft extends outside the chambers, thereby reducing the number ofseals required and consequently reducing the risk of leakage; inaddition that greatly simplifies machining and assembly of the fixed andmoving parts by reducing the number of orifices (bearings) through whichthe shaft passes and that must be in accurate alignment; this alsoreduces friction forces on the shaft and on the pistons, and increasesefficiency.

The structure of the apparatus also makes it possible to reduce themechanical stresses applied to the shaft; this structure makes itpossible to use elongate chambers, in particular chambers for which theratio of length over diameter is greater than or equal to 3, moreparticularly lies close to the range 5 to 10, or 10 to 20. This elongatetubular shape makes it easier to make the chamber-defining bodies whichmust be capable of withstanding high pressures. This also contributes toobtaining a flow rate that is continuously variable or constant, therebyeliminating (and/or considerably diminishing) transient surges (at theends of strokes), in particular by facilitating control over pistonspeed, and consequently over the speed of the common shafts.

These advantages are increased when at least one of the two end pistonsis not rigidly connected to the corresponding end of the sliding shaft,in particular when the piston is connected to the shaft by connectionmeans that allow the piston to move relative to the shaft about at leastone axis. In particular, the connection can be constituted by aball-and-socket joint or by a cardan joint, allowing relative rotationabout at least one transverse axis (e.g. perpendicular to thelongitudinal axis), by a bearing that allows relative translation alongthe longitudinal axis, or indeed by a combination of said connectionmeans. When the piston is not connected to the shaft, both it and theend of the shaft present facing faces for coming into contact (bearingagainst) each other: while water for filtering is being delivered underhigh pressure, the shaft transmits the force exerted by the drivingfluid on the central piston to said end piston via said face. While theend chamber is being filled by booster means (pump means) under lowpressure, the end piston “follows” the end of the shaft while remainingin contact via said bearing face under drive from the (low) pressureexerted by the water for filtering acting against its first (front)face. Under such circumstances, the means for guiding sliding of thepiston in the chamber are preferably integrated in the periphery of theend piston.

According to a characteristic of the invention, the ratio of thecross-section (relative to the common longitudinal axis of the chambersand of the shaft) of said first portion of the end chamber to thesection of said second portion of the end chamber is proportional(equal) to the conversion ratio of the filter, which is generally about20% to 75%. The diameters of the shaft and of the chambers of each ofthe pumps are selected so as to comply with this proportion.

The two end chambers are preferably identical and symmetrical about thecentral chamber receiving the drive piston and driven by the hydraulicfluid. The ducts connecting the pump to the filter are alsosubstantially symmetrical.

In another aspect, a water filter installation comprises at least twopumps as defined above, with their inlets and outlets connected inparallel, with the operation of the pumps being maintained at a phaseoffset and with the velocity thereof being controlled and/or monitoredto ensure that the accumulated total flow rate from the various pumps interms of liquid (water) admitted via the inlet and of pressurized liquid(water) delivered via the outlet is substantially constant (preferablyto within 10%, and in particular to within not more than 5%).

The invention preferably comprises two identical pumps whose shafts aredriven at speeds and at a phase offset that vary during a cycle, withthe phase difference being equal neither to zero nor to 180°, and withthe sum of the absolute values of the velocities of the two shafts beingsubstantially constant over time.

Given that the two water compression chambers of each pump operate inphase opposition by construction, adding a second pump in parallel withthe first and having its shaft moving with a phase offset (relative tothe movement of the shaft of the first pump) having a value that lies inthe range 10° to 170°, for example, makes it possible to ensure that theflow rate of the fluid to be filtered never passes through zero as wouldotherwise happen in the presence of a single pump whenever the shaft(and the three pistons associated therewith) of the pump reaches the endof a stroke.

According to a characteristic of the invention, a first of the twoshafts is accelerated while the second shaft is pausing at the end of astroke (dead center point). In addition, the portion of the end chambercan be connected to a source of fluid under pressure so as to enablesaid fluid to pressurize the water sucked in by the piston (and/ordelivered by an upstream booster pump) into said portion of the chamber,up to the normal feed pressure of the filter, thereby avoiding a(temporary) drop in the inlet pressure of the filter while this portionof the chamber is in communication with the inlet of the filter. Thistemporary pressurization is performed when the corresponding piston isat the end of a stroke (dead center position) after said portion hasbeen filled. For this purpose, this chamber portion can be momentarilyisolated from the suction and delivery circuits.

In a preferred embodiment, the supersalinated water (concentrate) isalso used for cooling the driving hydraulic oil, by passing through aheat exchanger.

According to other preferred characteristics of the invention:

the tubular water pumping chambers are selected to have a diameter lyingin the range 50 millimeters (mm) to 1000 mm, in particular in the range100 mm to 600 mm;

the maximum velocity of the shaft and the pistons is maintained to liein a range extending from 0.1 meters per second (m/s) to 10 m/s, andpreferably in the range 0.25 m/s to 3 m/s;

the shafts and the pistons are caused to perform a pause at each end ofthe chambers (“top” and “bottom” dead center points), in particular inorder to perform a step of pressurizing the water for a duration whoseratio relative to the shaft cycle period lies in the range 0.005 to 0.1,and in particular is approximately 0.01 to 0.05; and

a hollow shaft is used to reduce the inertia of the moving equipment andto reduce friction on the bearings.

The advantages obtained by the invention will be better understood onreading the following description which refers to the accompanyingdrawings, which drawings show preferred embodiments of the inventionwithout any limiting character.

In the drawings, elements that are identical or similar are given thesame references from one figure to another, unless stated to thecontrary.

FIG. 1 is a diagram of an installation for desalinating sea water andcomprising two identical pumps;

FIGS. 2 and 3 show the same installation in two different states of thepumping cycle.

FIG. 4 is a histogram showing the velocity of the shaft in each of thepumps of FIGS. 1 to 3, showing how these velocities vary during a cycle.

FIG. 5 is similar to FIGS. 1 to 3, showing a similar installation inwhich two-port solenoid valves are used to replace the distributorvalves of FIGS. 1 to 3.

FIG. 6 is a diagram showing means for maintaining pressurization in thechambers of a pump.

FIG. 7 is a fragmentary schematic of a variant embodiment of theinvention in which the installation comprises three pumps connected inparallel.

The installation 1 is for desalinating water delivered to an inlet 2 bya booster pump (not shown) at a pressure lying in the range 3 bars to 4bars. For this purpose, salt water is pressurized by each of twoidentical pumps 3, 4 to a pressure of 70 bars and it is delivered viaducts 5 to the inlet 6 of a reverse osmosis filter 7. The resultingfresh water is removed at 8 while the supersalinated water leaving thefilter 7 at 9 at a pressure of 69 bars is taken back to the pumps 3, 4by ducts 10. Inside the pump, the supersalinated water gives back itsenergy to the sea water which is about to be filtered, and is thenremoved at 11 under a pressure of 1 bar.

The extra energy required for pressurizing the sea water to be filteredup to 70 bars is supplied to each pump 3, 4 by a hydraulic unit 12 whichdelivers a flow of oil at substantially constant pressure and flow ratevia an outlet 13. The oil is conveyed to the pumps by a duct 14 andreturns to the hydraulic return of the unit via a duct 15.

With reference to FIG. 1 in particular, each of the pumps 3, 4comprises:

a body 16 defining three cylindrical chambers 18 a & 18 b, 19 a & 19 b,and 20 a & 20 b; these three tubular chambers are in alignment along anaxis 17 and they are separated from one another by two partitions 21, 22each pierced by an orifice fitted with a bearing having sealing gaskets;and

a hollow shaft 23 extending along the axis 17 and carrying three pistons24, 25, 26; the shaft is mounted to slide in translation (along arrows28) through the bearings fitted in the partitions 21, 22; the middle ofthe shaft 23 has a drive piston 25 suitable for sliding in the centralchamber 19 a, 19 b under the effect of the pressure which is applied toone of its faces by the oil inserted into the portion (or cavity) 19 aor on the contrary into the portion (or cavity) 19 b depending on thedesired travel direction, as a function of the position of a distributorvalve 27 connecting said chamber to the ducts 14, 15; the piston 24separates the portions 18 a and 18 b of a first end chamber while thepiston 26 separates the portions (or cavities) 20 a and 20 b of thesecond end chamber; the geometrical configuration of the movingequipment is symmetrical about a transverse midplane, as is theconfiguration of the chambers in the body 16.

In the state shown in FIG. 1, movement of the piston 24 of each pump 3,4 in the direction of arrow 28 causes the water for filtering that ispresent in the cavity 18 a of each pump to be delivered at 70 bars intothe ducts 5 leading to the filter 7 via respective distributor valves 29and 50. Simultaneously, water for filtering fills the end cavity 20 b ofeach pump, flowing along ducts 30, 31, 32. The energy required forcompressing the water in the cavity 18 a by means of the face 24 a ofthe piston 24 is supplied in part by the concentrate penetrating intothe cavity 18 b as delivered by the duct 10 and respective distributorvalves 51 and 52, the pressure of this concentrate acting on the secondface 24 b of the piston 24, and in part by the effect of thrust on thepiston 25 delivered by the oil penetrating into the cavity 19 b andcoming from the unit 12, which force is transmitted to the piston 24 bythe shaft 23.

The module and the direction and travel velocity of the two shafts arecontrolled by varying the positions (and/or states) of the respectivedistributors valves 27 associated with the two pumps 3, 4.

This regulation can be controlled electrically or hydraulically, byconventional means, not shown.

When the distributor valves 27 are in the position shown in FIG. 1, oildelivered into the duct 14 by the pump of the unit 12 is conveyed inpart into the cavity 19 b of the pump 3 via duct 33 b and in part intothe cavity 19 b of the pump 4 via duct 34 b. The flow rates of these twostreams of oil travelling respectively along the two ducts 33 b and 34 band which are functions of the positions of the distributor valve 27,are adjusted so as to cause the pump 3 to start (from its bottomdead-center point) and so as to ensure that the shaft of the pump 4travels at a velocity of 1 m/s. As shown in FIGS. 2 and 4, thedistributor valves are then controlled so as to increase the flow rateof oil along the duct 33 b while simultaneously decreasing the flow rateof oil along the duct 34 b until both flow rates are substantially inequilibrium (identical) so that they cause both shafts 23 to moveidentically at a velocity of 0.5 m/s (FIG. 2).

As shown in FIG. 4, the velocity of the shaft of each pump variesperiodically in reciprocating manner (mean value zero) with portions atconstant velocity. The mean velocity (in absolute value) of each pumpshaft is 0.5 m/s, and the sum of the magnitudes of the velocities of thetwo shafts is maintained at a value of 1 m/s, thereby causing sea waterto be admitted and pressurized sea water to be delivered at a constantflow rate. The three operating states shown in FIGS. 1 to 3 correspondrespectively to points on the graphs of FIG. 4 having abscissa values of0.7 seconds (s) (second points on the graphs); 0.8 s (third points onthe graphs); and 3.5 s (sixth points on the graphs).

FIG. 4 shows a pause at zero velocity lasting for 0.1 s is performed ateach dead point (stroke end). The graphs showing the velocities of thetwo pumps 3 and 4 are offset in phase by a value which varies during oneperiod about a mean phase offset value of the order of 1.2 s, i.e. 54°given that the cycle period has a value of about 8 s.

A heat exchanger referenced 80 in FIG. 7 is preferably connected to theduct 35 for removing super-salinated water at low pressure and to one ofthe ducts 14, 15 for transporting oil so as to cool the oil.

A proximity sensor 36 responsive to the piston 24 (e.g. anelectromagnetic sensor) is placed in the vicinity of the longitudinalend of each chamber 18 a, 20 b and is connected to a monitor unitreferenced 81 in FIG. 7 for controlling the valves.

During the pause, after the cavities 18 a, 20 b have filled with waterfor filtering, a pressurizing member is momentarily put intocommunication with the cavity so as to cause its pressure to go from 4bars to 70 bars. As shown in FIG. 6, this can be performed by using anexpansion chamber 43 (having a diaphragm 44) connected to the pumpchamber and to a supply 40 of oil under pressure via ducts provided withcontrolled isolating members 41, 42 (solenoid valves). An accumulator 82is connected to the recuperation cavity 18 b which receives thesuper-salinated water, thus enabling pressure variations in said cavityto be damped.

The pumping installation shown in FIG. 7 has three identical pumps 3, 4,and 60 which are connected in parallel via their inlets and outlets tothe water suction and delivery ducts (not shown) in the same manner asthat described above.

Unlike FIGS. 1, 2, 3, and 5, the hydraulic actuator 61 for driving eachpump is not disposed in the central portion of each pump but isseparated from the body defining the cavities for sucking in anddelivering water to be filtered (18 a, 20 b) and supersalinated water(18 b, 20 a).

Each actuator 61 comprises said drive piston 25 sliding in a cylindricalchamber 19 a, 19 b in alignment with the chambers of the water pumps 3,4, or 60 associated with the actuator, said piston 25 being connectedvia a rod 62 to the sliding shaft 23 of the pump. The rod (or secondaryshaft) 62 is mounted to slide relative to the body of the actuator 61and the associated pump via leakproof bearings such as 63 providedthrough the walls of said bodies. In order to ensure that the ratios ofthe cross-sections of the cavities 20 a, 20 b is identical to that ofthe cross-sections of the cavities 18 b, 18 a, a rod 64 of sectionidentical to that of the rod 62 is fixed to the piston 26 and is mountedto slide through an orifice pierced in the body of the pump via aleakproof bearing 65. The rods 62 and 64 are in alignment, as is theshaft 23 on the longitudinal axis 17 common to the pump and to theactuator, which axis is preferably horizontal so that the weight of themoving equipment (rods, pistons, and shaft) in each pump does notcomplicate controlling the movement thereof.

The loop 66 common to the three pumps for providing oil under pressureto drive the three actuators 61 includes a pump 12 delivering into aduct 14 fitted with a flow meter 67, and a duct 15 for returning oil toa return 68, with the cooler 80 being fitted to said duct.

Oil pressurized by the pump 12 is conveyed by the duct 14 to the inletof a distributor valve 68 whose return outlet is connected to the duct15.

The distributor valve 68 distributes the flow of oil as delivered by thepump 12 to the actuator 61 for actuating the pumps 3, 4, and 60 asdescribed above, under the control of the control unit 81 which receivessignals from the sensors 36 and 67.

For this purpose, each double-acting actuator 61 is connected to thedistributor valve 68 via two ducts 69, 70.

What is claimed is:
 1. Water pumping apparatus comprising first andsecond pumps (3, 4, 60), each of the pumps having two chambers (18 a, 18b, 20 a, 20 b) in alignment and receiving respective pistons (24, 26)movable in translation within the cambers, the pistons beinginterconnected by a sliding shaft (23), double-acting, hydraulicactuators (19 a, 19 b, 25, 61) respectively for driving the pumps and aloop (14, 15) for circulating driving hydraulic fluid therefrom, theloop being common to the hydraulic actuators, and valve means (12, 27,68, 81) for pausing the sliding shaft and pistons of the first of thepumps for a prolonged period at an end of a stroke thereof, acceleratingthe sliding shaft and pistons of the second of the pumps while the firstof the pumps is paused, and selectively putting each of the actuatorsinto communication with the loop in such a manner that the sum ofabsolute values of hydraulic fluid flow velocities to the actuators forthe driving is substantially constant.
 2. Water pumping apparatuscomprising at least two pumps (3, 4, 60), each pump comprising: at leasttwo chambers (18 a, 18 b, 20 a, 20 b) in alignment on a longitudinalaxis (17); at least two pistons (24, 26) respectively mounted to move inreciprocating translation in each of the two chambers; and atransmission shaft (23) for transmitting forces between the two pistons,which shaft extends in part in each of the chambers and is mounted toslide relative thereto along said longitudinal axis; the pumpingapparatus further comprising an actuator (19 a, 19 b, 25, 61) suitablefor delivering the energy required for compressing the water, minus theenergy recovered from the concentrate by said pistons, by causing theshaft (23) to move in reciprocating translation (sliding) together withthe pistons in each of the pumps, and means (27, 68, 81) for causing theshaft and the pistons of each pump to pause for a prolonged period atthe end of each stroke, and also means (27, 68, 81) for accelerating oneof said two pumps while another one of said pumps is performing aprolonged end-of-stroke pause, and control means to maintain the totalof flow velocities of water discharged by the pumps substantiallyconstant.
 3. Water pumping apparatus, characterized in that itcomprises: two pumps (3, 4, 60) connected in parallel, each pump havingtwo pistons (24, 26) mounted to move in translation in two alignedchambers (18 a, 18 b, 20 a, 20 b), and a sliding shaft (23) fortransmitting forces between the pistons, in which a rear face (24 b, 26b) of each of the two pistons (24, 26) co-operates with the pump body(16) and the shaft (23) to define a cavity (18 b, 20 a) receivingconcentrate under pressure for contributing to pressurizing the water ina cavity (18 a, 20 b) defined by said body and the front face (24 a, 26a) of the piston (24, 26); an actuator (19 a, 19 b, 25, 61) associatedwith each pump to drive it; and actuator control means (12, 27, 68, 81)for causing the shaft and the pistons of each pump to pause for aprolonged period at the end of each stroke and for accelerating one ofsaid pumps while another one of said pumps is pausing for a prolongedperiod at the end of its stroke, thereby enabling a phase offset to becontinuously maintained between the movements of the two pumps with thevalue of said offset being neither zero nor 180° and for maintaining allsums of absolute values of velocities of the slidings of the shafts (23)substantially constant.
 4. Apparatus according to claim 1, furthercomprising means (40 to 44, 82) for pressurizing the water pumpingchambers (18 a, 18 b, 20 a, 20 b) when the shaft and the pistons (17,24, 26) operating in the water pumping chambers (18 a, 18 b, 20 a, 20 b)are motionless, and more particularly during the powering of theapparatus and/or when the said shaft and pistons (17, 24, 26) are at theend of each stroke.
 5. Apparatus according to claim 1, furthercomprising control means (12, 27, 68, 81) for maintaining the sum of theabsolute values of the velocities of the shafts (23) to a value which issubstantially constant.
 6. Apparatus according to claim 1, wherein theloop comprises a hydraulic unit common to the pumps.
 7. Apparatusaccording to claim 1, in which the pistons (24, 26) are identical andare disposed at opposite ends of the shaft (23).
 8. Apparatus accordingto claim 1, in which a third piston (25) is fixed to the shaft (23) atequal distances from the other two pistons (24, 26), which third inpiston is slidable in a chamber (19 a, 19 b) for receiving a drivinghydraulic fluid, and in which the moving equipment (23 to 26) and thechambers (18 a to 20 b) are disposed symmetrically about a transversemidplane.
 9. Apparatus according to claim 1, in which the chambers (18a, 18 b, 20 a, 20 b) are tubular and elongate, with the ratio of chamberlength over diameter being greater than or equal to
 3. 10. Apparatusaccording to claim 1, in which the neither of the two pistons (24, 26)is rigidly connected to the corresponding end of the shaft (23). 11.Apparatus according to claim 1, in which both pistons (24, 26) areconnected to the corresponding shaft end (23) via means comprising aball-and-socket joint, a cardan joint, or a sliding bearing.
 12. Amethod of desalinating sea water by reverse osmosis, in which apparatusaccording to claim 1 is used and in which the maximum velocity of theshafts and pistons is maintained a value lying in the range 0.1 m/s to10 m/s.
 13. A method of desalinating sea water by reverse osmosis, inwhich apparatus according to claim 1 is used, and in which a phaseoffset is maintained between two pumps (3, 4, 60) at a value lying inthe range 10° to 17°.
 14. A method of desalinating sea water by reverseosmosis, in which apparatus according to claim 1 is used, and in whicheach pump is caused to move periodically with a period of duration lyingin a range 1 second to 100 seconds.
 15. A method according to claim 14,in which a pause is caused at the end of the stroke of each of saidpistons (24, 26) for a duration whose ratio compared with the saidperiod has a value lying in the range 10⁻³ to 10^(—1).
 16. Apparatusaccording to claim 2, further comprising means (40 to 44, 82) forpressurizing the water pumping chambers (18 a, 18 b, 20 a, 20 b). 17.Apparatus according to claim 2, further comprising control means (12,27, 68, 81) for maintaining the sum of the absolute values of thevelocities of the shafts (23) to a value which is substantiallyconstant.
 18. Apparatus according to claim 2, further comprising ahydraulic unit (12) common to the pumps (3, 4, 60).
 19. Apparatusaccording to claim 2, in which the pistons (24, 26) are identical andare disposed at opposite ends of the shaft (23).
 20. Apparatus accordingto claim 2, in which a third piston (25) is fixed to the shaft (23) atequal distances from the other two pistons (24, 26), which third pistonis slidable in a chamber (19 a, 19 b) for receiving a driving hydraulicfluid, and in which the moving equipment (23 to 26) and the chambers (18a to 20 b) are disposed symmetrically about a transverse midplane. 21.Apparatus according to claim 2, in which the chambers (18 a, 18 b, 20 a,20 b) are tubular and elongate, with the ratio of chamber length overdiameter being greater than or equal to
 3. 22. Apparatus according toclaim 2, in which the neither of the two pistons (24, 26) is rigidlyconnected to the corresponding end of the shaft (23).
 23. Apparatusaccording to claim 2, in which both pistons (24, 26) are connected tothe corresponding shaft end (23) via means comprising a ball-and-socketjoint, a cardan joint, or a sliding bearing.
 24. Apparatus according toclaim 1, further comprising means (80) for cooling the hydraulic fluidwith water.
 25. Apparatus according to claim 2, further comprising means(80) for cooling the hydraulic fluid with water.